1. Technical Field
The present invention generally relates to a method and system for processing viscous liquid crude hydrocarbons.
2. Description of the Related Art
As world reserves of light, sweet crudes diminish and worldwide consumption of oil increases, refiners seek methods for extracting useful oils from heavier crude resources. Extensive reserves in the form of “heavy crudes” exist in a number of countries, including Western Canada, Venezuela, Russia, the United States, and elsewhere. For example, heavy or extra heavy crude oil can be found in the Orinoco Belt in Venezuela, the oil sands in Canada, and the Ugnu Reservoir in Northern Alaska. Alberta produces approximately two-thirds of Canada's oil and more than three-quarters of its natural gas. Nearly half of Alberta's oil is mined from vast oil sands, which contain deposits of a heavy crude oil called bitumen. Alberta's oil sands represent the largest known deposits of bitumen in the world. The oil sands occur in three major areas of the province: the Athabasca River Valley in the northeast, the Peace River area in the north, and the Cold Lake region in east central Alberta.
The heavier crudes, which can include bitumens, heavy oils and tar sandspose processing problems due to significantly higher concentration of contaminants such as sulfur and nitrogen as well as metals, most notably iron, nickel and vanadium. Bitumen is more costly to mine than conventional crude oil, which flows naturally or is pumped from the ground. This is because the thick black oil must be separated from the surrounding sand and water to produce a crude oil that can be further refined. The bitumen, which contrary to normal crude found in a deep reservoir, does not have the same light fractions normal crude. The bitumen thus consists of heavy molecules with a density exceeding 1.000 kg/dm3 (less than 10 API gravity) and a viscosity at reservoir conditions 1000 times higher than light crude. Because of the composition of the bitumen, it has to be upgraded before it can be refined in a refiner as light crude.
The large reserves of heavy or extra heavy crude oil are very viscous in their natural state. The viscous nature of the crude oil, however, makes it difficult to transport the oil in conventional pipelines to stations where it can be processed into useful end products. The origin of high viscosity in these oils has been attributed to high asphaltene content of the oils. Asphaltenes are organic heterocyclic macromolecules which occur in crude oils. Under normal reservoir conditions, asphaltenes are usually stabilized in the crude oil by maltenes and resins that are chemically compatible with asphaltenes, but that have lower molecular weight. Polar regions of the maltenes and resins surround the asphaltene while non-polar regions are attracted to the oil phase. Thus, these molecules act as surfactants and result in stabilizing the asphaltenes in the crude. However, changes in pressure, temperature or concentration of the crude oils can alter the stability of the dispersion and increase the tendency of the asphaltenes to agglomerate into larger particles. As these asphaltene agglomerates grow, so does their tendency to precipitate out of solution.
Generally, unwanted asphaltene precipitation is a concern to the petroleum industry due to, for example, plugging of an oil well or pipeline as well as stopping or decreasing oil production. Also, in downstream applications, asphaltenes are believed to be the source of coke during thermal upgrading processes thereby reducing and limiting yield of residue conversion. Viscosity reduction of heavy oils is therefore important in production, transportation and refining operations of the oil. Accordingly, transporters and refiners of heavy crude oil have developed different techniques to reduce the viscosity of heavy crude oils to improve its pumpability.
One solution has been to form oil-in-water emulsions. Oil-in-water emulsions exhibit greatly reduced viscosity which facilitates its transport through a pipeline. For example, U.S. Pat. No. 4,392,944 (“the '944 patent”) discloses a stable oil-in-water emulsion of heavy crude oil and bitumen and subsequent breaking of the emulsion. The '944 patent discloses that the emulsion can be broken by conversion of the oil-in-water emulsion into a water-in-oil emulsion using calcium hydroxide (i.e., slaked lime or hydrated lime) and dewatering of the resulting water-in-oil emulsion. Another example is U.S. Pat. No. 5,526,839 which discloses a method for forming a stable emulsion of a viscous crude hydrocarbon in an aqueous buffer solution, involving the steps of (a) providing a viscous crude hydrocarbon containing an inactive natural surfactant; (b) forming a solution of a buffer additive in an aqueous solution to provide a basic aqueous buffer solution, wherein the buffer additive activates the inactive natural surfactant from the viscous crude hydrocarbon; and (c) mixing the viscous crude hydrocarbon with the aqueous buffer solution at a rate sufficient to provide a stable emulsion of the viscous crude hydrocarbon in the aqueous buffer solution.
Another solution has been the use of ultrasonic irradiation to alter the asphaltene fraction. For example, U.S. Patent Application Publication No. 2004/0232051 discloses a process of sonicating a starting heavy oil in the presence of an acid selected from the group consisting of mineral acids, organic acids and mixtures thereof in the absence of hydrotreating conditions to produce a decreased viscosity heavy oil composition comprising a dispersed phase of asphaltene salts of acids wherein the acids are selected from the group consisting of mineral acids, organic acids, and mixtures thereof in a hydrocarbon continuous phase.
Yet another solution is the use of dispersants to disassemble or break up the agglomerates of asphaltenes in the oil. For example, U.S. Pat. No. 6,187,172 discloses a method for dispersing asphaltenes in a liquid hydrocarbon by incorporating into the liquid hydrocarbon a sufficient concentration, e.g., about 0.1 to about 25 weight percent, of a hydrocarbon soluble asphaltene dispersant.
Asphaltene-containing liquid crude hydrocarbon feedstocks which are unacceptable for transportation impart a low economic value to the unacceptable feedstock. Accordingly, it would be desirable to provide improved methods and systems for processing and transporting asphaltene-containing viscous liquid crude hydrocarbons that can be carried out in a simple, cost efficient manner.